The present invention relates to a hydraulic capsule control system during the rolling cycle of tubes, bars, and rod-like bodies in general, in rolling systems.
State of the Art
Rolling mills for the longitudinal rolling of tubes, and rod-like bodies in general, comprise groups of rolling stands with 2, 3 or more rollers per stand. The rollers of each stand are held together by a cartridge, which makes fitting and removing the rollers easier. In the known rolling mills, the working cartridges are changed in direction either parallel to the rolling axis or transversal thereto. In the latter case, the cartridges are thus changed laterally with respect to the rolling stands, and specifically, in systems in which the hydraulic capsules for regulating and controlling the rolling pressure are rigidly fixed to the outer frame of the stand, capsule piston stroke lengths are provided so as to make the pistons of the capsules retract outside the clearance constituted by the trajectory traveled by the roller holder cartridge during the side extraction of the same from the rolling mill. Such releasing strokes may vary according to the maximum diameter of the tube which can be manufactured by the rolling mill with values indicatively included from 150 to 400 mm, the minimum value being referred to rolling mills for 4″½ tubes, the higher value to rolling mills for 20″ tubes. Experience in rolling shows that such values cause problems to the hydraulic capsule position control system during the entire rolling of the tube, but more specifically during the transient steps of leading-in and unloading of the tube from each single stand, when the pressure conditions in the main chamber and in the annular chamber of the hydraulic capsule suddenly change, passing from a discharged condition to a charged condition, and vice versa during unloading. The quality of the regulation of the roller position, and specifically the capacity of the control system to very rapidly correct the movements of the rollers as the loads acting thereon change, greatly depends on the physics of the system governed by the capsule piston stroke. It is known that the physical system becomes more elastic as the capsule stroke increases; the chambers which contain the hydraulic oil being larger, it is consequently more difficult to control oscillations and vibrations of the piston position in the capsule, particularly during transient steps. In the prior art, based on approximately 20 years of use of capsules with stroke shorter than 150 mm, three-way servo valves are used (FIG. 4), the pressure and discharge of which are connected only to port A, being the latter connected to the main chamber of the hydraulic capsule. Port B of the servo valve is closed and the annular chamber is fed by valve systems adapted to attempt to guarantee a pressure as constant as possible in the annular chamber itself. If, as in the case of WO2011/132094, the stroke of the capsule reaches 300 mm or more, up to 400 mm, devices must be evaluated to avoid drastically worsening system functionality with evident repercussions on final product quality consequent to capsule strokes longer than those normally used of 120-160 mm. It is therefore felt the need to make a control system for hydraulic capsules aimed at reducing duration and entity of the error during transient steps and which allows to overcome the aforesaid drawbacks.